Use a Sling for Your Autoblock
Autoblock knots are easy and fast to tie. To tie an autoblock knot, you need either a short length of thin cord or a nylon sling. The knot can, however, be tied in an emergency situation with any piece of cord or webbing you might have on you. I’ve even seen it tied with the cord threaded on a Hexentric nut. Many climbers use a two-foot, shoulder-length, 9/16-inch-wide sling for their autoblock since it’s a common piece of gear that is always carried when climbing. It’s best to use a nylon rather than a Spectra sling. Also use narrow webbing rather than one-inch-wide webbing.
Use Cord for Your Autoblock
Other climbers, like myself, use a piece of cord attached to a carabiner that is carried specifically for tying the autoblock. Use a thin cord (best if its 5mm or 6mm in diameter) that, after the ends are tied together with a double fisherman’s knot forming a closed loop, is 18 inches long. You’ll need a 48-inch length of cord to make this loop. Remember that the thinner the cord, the greater the bite it will have on the rappel rope but the faster it will wear out. Also remember that since this cord is meant to be loaded, it is possible for the double fisherman's knot to lose its tails, that is the tail can slip into the knot and it can come undone. Always make sure you have two-inch tails on the knot. Tape the tails to the cord and you will see if slippage occurs.
You need either a thin cord or nylon sling to tie your autoblock knot.
Check the Cord for Wear
It’s very important that you regularly check your autoblock sling or cord for wear and tear. I always look at mine after every long rappel to make sure it’s not getting too worn. Look for stitching starting to unravel on sewn slings and for wear from sliding down the rope. When it’s looking worn, retire it and use a new one.
